[FrameworkBundle] Extract KernelTestCase from WebTestCase

| Q             | A
| ------------- | ---
| Bug fix?      | No
| New feature?  | Yes
| BC breaks?    | No
| Deprecations? | No
| Tests pass?   | Yes
| Fixed tickets | None
| License       | MIT
| Doc PR        | symfony/symfony-docs#3311

Previous discussion is in #7704. Opened new PR to target master branch.

This idea came to me while reading the Cookbook article [How to create a Console Command - Testing Commands](http://symfony.com/doc/master/cookbook/console/console_command.html#testing-commands) and I wanted to hear feedback from others before submitting a patch.

Basically the Cookbook article states that when testing a Command that extending `WebTestCase` is a good way to get access to the Kernel and thus the Container. This struck me as weird because I was wanting to test a Command, which doesn't really have anything to do with "the web."

Currently `WebTestCase` doesn't do anything internally that looks like web-specific work, and the class docblock itself states "WebTestCase is the base class for functional tests.".

After a suggestion in #7704 by @beberlei, I decided to take his advice and now have the following implementation:

Extracted a new class `KernelTestCase` from `WebTestCase` and instead allow WebTestCase to extend KernelTestCase. Pulled all methods up into KernelTestCase except `createClient()` because `createClient()` is focused solely on creating a Client for issuing web-based requests.

Benjamin's solution provides us a clear extension point from `KernelTestCase` for Command-based tests and also provides 100% backwards-compatibility without the need to deprecate WebTestCase.